FIG. 2 shows a conventional plunger pump 10 including a pump head 11 and a pump body 12 which are connected to each other via a connector part 13. In the pump head 11, a cylindrical passage is formed, which is referred to as a cylinder 15, wherein a plunger 16 is inserted. Further, a suction passage 17 and a discharge passage 18, both connected to the cylinder 15, are provided in the pump head 11, and a suction check valve 19 and a discharge check valve 20 are disposed in the above passages, respectively.
The plunger 16 extends through the connector part 13 into the pump body 12, where the rear end of the plunger 16 is fixed to a plunger holder 21 having a diameter larger than that of the plunger 16. The plunger holder 21 is accommodated in a cavity formed at the front end of a shaft 22 which can move back and forth in the pump body 12, where the plunger holder 21 is held at its front and rear ends. The front and rear ends of the plunger holder 21 are spherical, whereas the inner face of the cavity for retaining the rear end of the plunger holder 21 is flat. Thus, even when the plunger 16 is held in the cavity, it still has a degree of freedom, Ftr, to move transversely, i.e. in the direction vertical to the central axis, and another degree of freedom, Fro, to rotate, or swivel.
The shaft 22 is driven back and forth by a cam 25 and a spring 26, where the shaft 22 is regulated by a cam follower 27 provided at the rear end of the shaft 22 and a bearing 28 provided in the pump body 12 so that it can move only in the direction of its axis.
The space between the front part of the plunger 16 and the pump head 11 and the space between the rear part of the plunger 16 and the connector part 13 are sealed by seals 30 and 31, respectively. Leakage of a liquid from the cylinder 15 is prevented by the seal 30, and leakage of the a plunger-washing liquid, which is supplied through a plunger-washing passage 32 when necessary, is prevented by the seals 30 and 31.
Since the plunger 16 is held by the two seals 30, and 31 at its front and rear parts, respectively, the plunger 16 is bound to move on the straight line passing through both the centers of the seals 30 and 31. On the other hand, the shaft 22 is bound to move in the direction of the axis determined by the bearing 28 as described above. In assembling the plunger pump 10, the axis of the plunger 16 and the axis of the shaft 22 may be displaced transversely from each other and/or inclined to each other. In these cases, not only the plunger 16 moves incorrectly, but also such an excessive force may be transmitted to the seals 30 and 31 that results in damage to the seals. In view of this, the contacting part of the plunger 16 (i.e. its rear end) is shaped spherical while the contacting part of the shaft 22 (i.e. the inner face of the cavity of the shaft 22) is flat, so that the degrees of freedom for the plunger 16 to move in the traversal direction and to rotate, Ftr and Fro, are provided as described above.
When the plunger holder 21 is set in the cavity of the shaft 22, a small gap of about several tens of .mu.m is left at the front and/or rear ends of the plunger holder 21, so that the plunger 16 has the degrees of freedom in motion and rotation as described above. Due to the gap, however, a time lag occurs between the motion of the plunger 16 and the actual suction and discharge of liquid by the pump 10, which causes various problems such that the pumping pressure fluctuates during the pumping operation, for example. Particularly, when a low pressure gradient elusion is carried out, the flow rates of solvents pumped out by different pumps, respectively, cannot be regulated correctly, so that the actual concentration of the mixture of the solvents at a time point cannot be equal to an objective concentration predetermined corresponding to the time point.
When the pump 10 is assembled, a small displacement or inclination is allowed between the axis of the plunger 16 and the axis of the shaft 22, as described above. Here, however, the inclination between the two axes prevents the shaft 22 and the plunger 16 from moving smoothly during the operation of the pump 10. The reason is explained as follows. Referring to FIG. 3, when the shaft 22 thrusts the plunger holder 21 with its axis 52 being inclined to the axis 53 of the plunger 16, the contacting point where the flat end of the shaft 22 contacts with the spherical end of the plunger holder 21, moves from the point P0 to the point P1. When the contacting point moves, an excessive, force is worked on the seals 30 and 31 by the plunger 16 due to the friction between the shaft 22 and the plunger holder 21, which may cause damage to the seals 30 and 31 and/or leakage of the liquid.